1. Field of the Invention
The present invention relates to a next generation mobile terminal, and more particularly to a system and method for performing cell selection in a terminal of a mobile communications system.
2. Background of the Related Art
A public land mobile network (PLMN) is a network created for the purpose of providing land mobile communication services. Generally, PLMN is restricted by geographical borders, and at least one PLMN may exist in every country according to the international rules and regulations. If several PLMNs exist in one country, areas of these PLMNs often overlap, and areas of PLMNs may also overlap in the borders between different countries.
In order to provide service, a terminal that is moving must select a serviced PLMN whenever necessary. This usually occurs as follows. When a mobile terminal (MT) is switched on, it attempts to make contact with a PLMN. A particular PLMN is selected either automatically or manually. Once the PLMN is selected, the MT looks for a suitable cell within the chosen PLMN and chooses that cell to provide available services. This choosing is known as “camping on the cell.” If the MT subsequently loses coverage of the chosen cell, it reselects a most suitable alternative cell of the selected PLMN and camps on that cell. And, if the MT loses coverage of a PLMN, either a new PLMN is selected automatically or an indication of which PLMNs are available is given to the user, so that a manual selection can be made.
Thus, the MT carries out PLMN selection and cell selection/reselection from the point it is switched on, and finally selects a current serving cell. The cell selection process maybe classified largely into normal cell selection and stored list cell selection.
The normal cell selection is a process of initial cell selection, and is carried out when the MT is switched on. More specifically, the normal cell selection is a process in which the MT searches all radio frequency (RF) carriers received without previous information, compares them with each other, and selects a cell having the maximum power as a current serving cell.
The stored list cell selection is a process in which the MT periodically and successively measures signal quality of cells, and updates a profile list of one or more neighbor cells based on provided cell information. Later, the stored list cell selection allows the MT to select a serving cell using cell information, e.g., scrambling codes known from signal quality of respective cells and a neighbor cell profile list previously measured and stored. In particular, in the stored list cell selection process, the stored list represents IDS of PLMNs that can permit roaming of the MT and IDS of cells in which camping of the MT has been performed. In other words, a scrambling code that is inherently assigned to each cell is used as an ID, and this ID is stored.
In the foregoing discussion, the serving cell indicates a cell providing the MT with communication service at a present time, and a neighbor cell indicates cells adjacent to the serving cell. The neighbor cells include at least one available cell that is serviceable and at least one non-available cell that is non-serviceable.
In systems using the conventional stored list cell selection, if a PLMN ID is changed by request of the MT user or as a result of a change in an available condition of the current serving cell, it is necessary to again perform the process of the initial cell selection.
The following table 1 represents stored list for the cell selection in the communication network shown in FIG. 1, in which PLMN ID-1, PLMN ID-2 and PLMN ID-3 that are information of roaming-capable PLMN, and Cell ID-1, Cell ID-2 and Cell ID-3 that are neighbor cell information pertaining to the respective PLMNs are stored.
TABLE 1Stored ListPLMN ID-1PLMN ID-2PLMN ID-3Cell ID-1Cell ID-1Cell ID-1Cell ID-2Cell ID-2Cell ID-2Cell ID-3Cell ID-3Cell ID-3
In a state where this list is stored, it is assumed that the MT is positioned at an overlapped area of Cell 2 of PLMN 3 and Cell 3 of PLMN 2 as shown in FIG. 1.
When selecting the serving cell, the MT utilizes the stored list. Then, when it is assumed that the stored list is assigned priority in the order of PLMN IDS shown in table 1, a cell search for the cell selection is sequentially carried out from Cell 1 of PLMN 1.
When it is assumed that the MT camps on Cell 3 of the PLMN 2 through the aforementioned cell selection process, if the available condition of the PLMN 2 is later changed or if the signal strength of the PLMN 2 does not reach a prescribed threshold and thus the signal quality is non-available, there may occur a case in which Cell of other available PLMN has to be searched or a case in which user of the MT wishes to reselect the PLMN 3. In such a case, the MT has to again start the search from the Cell 1 of the PLMN 1, of which priority is prescribed as shown in table 1.
The MT does not camp on the Cell 2 of the PLMN 3 until it again measures signal strengths for the Cells of the PLMN 1, the Cells of the PLMN 2 and the Cell 1 of the PLMN 3. In other words, it is necessary to again perform the process of the initial cell selection from the very beginning.
In a final step, the conventional stored list cell selection method utilizes IDS of PLMNs that can permit the roaming of the MT, and IDS of cells that the MT was previously camped on.
After the MT camps on a cell of a specific PLMN, if there occurs a case in which the available condition of PLMN on service is changed into non-available condition or user of the MT is willing to reselect not current PLMN on service but other PLMN, the process of initial cell selection most again be performed. The conventional stored list cell selection method therefore has a drawback in that it takes much time for the MT to reselect the cell.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.